SEEPAGE CUT-OFFS FOR LEVEES: A ... - Geosystems, L.P.

Figure 3. Typical arrangement of a backhoe wall
(Soletanche Promotional Information).
to the trench may be required, together with tremie placement. Where CB is used, of
course, its dual purpose is to support the excavation and then to harden in place as the
backfill material. For SC and SCB walls, good technique involves bringing the toe of the
backfill close up to the excavated face after completion of the day’s work. The following
morning, the bottom of the trench is “cleaned” (most effectively by the excavator) and a
portion (say 2-5 feet) of the previous day’s backfill dug out of the trench to assure that no
highly permeable “stripes” of settled sediment are left in situ. It is typical to require a 50-
to 150-foot separation between backfill toe and base of excavation slope during routine
work, although there seems little engineering logic for this.
Most backhoe cut-offs for dams and levees have been 30-36 inches wide and not more
than 60 feet deep. However, recent developments have pushed maximum “comfortable”
depths to around 75 feet, while equipment has been developed to excavate to over 100
feet in favorable conditions.
Backfill Materials and Properties
The authors recommend that detailed guidance for the design and performance of the
various types of backfill mixes be obtained primarily from the classic texts, as referenced
above (and especially Millet et al., 1992), and from the other case history specific
accounts referenced in Bruce et al. (2006). As general background, however, the
following summary is provided.
CB (Self-Hardening Cement-Bentonite)
There is a very wide range in the relative components of these mixes, but in general such
mixes can be expected to comprise 3-4% bentonite and 15-30% cement. It is common to
include a retarder, while it is often overlooked that the mix in situ may well contain up to
10% or more of the native soil. An example of a mix used by Trevi as a “plastic” cut-off
for a dam in North Africa comprised:
– Bentonite: 45-50 kg/m 3 of mix
– Cement: 200-230 kg/m 3 of mix
– Water: 900-950 kg/m 3 of mix